The present disclosure is directed to a temperature sensor and more particularly to a solid state temperature sensor which responds to temperature in a sensitive fashion, thereby providing an apparatus yielding output voltage as a function of temperature. The output voltage can be adjusted to a sensitivity where a few microvolts represents a fraction of a degree. Sensitivities as small as 0.001.degree. F. are thought to be obtained with the present apparatus.
Precision temperature measurements typically are obtained with a quartz thermometer or other devices. The accuracy and precision of quartz devices is higher than other types of precision sensors. Useful resolution of 0.001.degree. F. is traceable to the National Bureau of Standards (NBS) and provides sensitive measurements which are relatively stable and repeatable. Such quartz systems however are typically quite expensive. A very popular model is the HP 2804A quartz thermometer, a very popular laboratory standard. Another device which finds acceptance as a precision sensor is the LM34 series provided by National Semiconductor Corporation which utilizes an integrated circuit package in a transistor can or a similar TO92 plastic package. Such devices have a nominal output of 10 MV/.degree. F. and have a published typical accuracy of .+-.0.4.degree. F. Platinum Resistive Temperature Devices (RTD) have a typical accuracy of .+-.0.1.degree. F.
Utilizing precision voltage measuring instruments the proposed sensor can be read to output voltages where the least significant digit represents about 0.00076.degree. F. in laboratory conditions, and is thought to be approximately equal to or better than the quartz thermometer mentioned above which exemplifies competitive devices with the temperature sensor disclosed herein.
The present apparatus however is an entirely different type of temperature sensor. It preferably utilizes a silicon planar PIN photodiode. Preferably the photodiode is mounted on a two lead transistor can, and one common size can be as large as a TO-18 package. Another device is the Siemens BPX 65 and representative device is the BPX 66 where the two devices differ only slightly in characteristics which are not significant. There are however even smaller devices. One particularly advantageous PIN photodiode is ordinarily sold as a combination with a laser where the laser and photodiode are mounted on a common base and the photodiode is positioned adjacent to the laser to monitor the backside emission of the laser. One of the features of the present disclosure is the use of a photodiode to provide a controllable laser output beam subject to control of the present apparatus.
Recalling the present disclosure is directed to a temperature sensor, one important use of this sensor is to measure the temperature of the laser source so that the laser performance can be properly controlled. The precise intensity of the light emission from a laser is subject to a number of variables including the laser pumping current. Another variable is laser device temperature. If certain measurements are made using the laser, they cannot be made accurately if the temperature of the laser is subject to drift or variation. It is desirable therefore to know the temperature of the laser so that stabilization of the laser parameters can be normalized with respect to a reference temperature. It is customary to obtain a laser with a monitor PIN photodiode. Ordinarily, that is connected in an optical feedback control loop which modulates the current drive for the laser from the laser power supply. That is well and good; however, it ignores the possibilities of temperature drift, and in particular the possibility that temperature drift may be crucial to the measurement system involving this particular laser. The present disclosure contemplates an alternate and secondary use of the monitor PIN photodiode so that the photodiode is able to measure the temperature of the laser and the surrounding structure for the express purpose of providing an indication of temperature stabilization, or the absence thereof.
The PIN photodiode of the present disclosure thus finds a second use as a temperature sensor. In conjunction with a timing circuit, it is switched out of the back illumination feedback path involving the laser current power supply and is then used in another circuit to provide an input signal indicative of temperature drift. To this end, the photodiode is mounted on a supportive substrate or base. It is normally mounted in common with the laser so that the two are jointly at the same operating temperature. That is, there is no temperature differential between the two of them. Mounting the laser and the PIN photodiode on a common substrate is helpful to provide some measure of output stabilization. The photodiode is connected in a circuit which provides a forward conduction current through the photodiode, this being regulated to a very accurate level, and wherein the regulated current is used to establish a null condition. Then, with temperature variation, the PIN photodiode forms a variation in DC voltage levels which is transferred by means of appropriate operational amplifiers connected as comparators and to otherwise provide nulling of the system, and ultimately forming an output signal. The output signal can be calibrated quite nicely to provide a few microvolts output per 0.001.degree. F., an extraordinary sensitivity for what is in retrospect a relatively simple device.
While the foregoing describes the temperature sensor, it additionally has the second use as mentioned earlier, namely, that the PIN photodiode is installed to respond to illumination periodically and is further arranged to form an easily repeatable temperature measurement. Many objects and advantages in addition to those named above will be observed on a review of the drawings and written specification found below.